The eye's most important focusing lens is the cornea, the transparent dome-shaped front part of the eye. The cornea must have a perfectly smooth surface in order to provide clear vision. If the corneal surface is irregular, such as from disease or injury, the eye can no longer focus clearly, even with the strongest glasses. If a person has an irregular cornea, hard corneal contact lenses can improve his or her vision by creating a smooth layer of tears that fills in the surface irregularities of the cornea. A hard corneal contact lens rests on and slides over the cornea, and creates friction between the lens and the cornea. Many people with damaged corneas cannot be fitted with a hard corneal contact lens because they cannot tolerate the discomfort and pain caused by this friction.
A scleral lens is a rigid contact lens that rests on the sclera of the eye; some scleral contact lenses rest on the peripheral surface of the cornea while others do not touch the cornea at all. The scleral lens defines a compartment of fluid between the inner surface of the scleral lens and the cornea, referred to herein as the “fluid compartment.” The portion of the scleral lens that contacts the sclera is referred to as the “scleral portion” of the scleral lens, or as the “haptic.” The portion of the scleral lens that covers the cornea is called the “optic portion” of the scleral lens. The optic portion may optionally be corrective. The greatest distance between the surface of the cornea and the posterior portion of the optic portion is at least 30μ. As used herein, the posterior side of the scleral lens is the side that is disposed on the eye.
Scleral lenses are advantageous for patients with corneal disease because they can avoid all contact with the diseased cornea, while the fluid compartment bathes the cornea in artificial tears. One issue with scleral contact lenses is that the lens can start to adhere to the cornea. This problem occurs because mucin, a glue-like material produced by the surface cells of the cornea, can accumulate and over time and fill the space between the cornea and the scleral lens. Under these circumstances, mucin adheres to the opposing surfaces of the scleral lens and cornea and gradually shrinks in volume, thus pulling the rigid lens against the cornea with sufficient compression to cause discomfort. As mucin production continues, the mucin becomes compacted by the pressure of the lens. This pressure squeezes aqueous fluid out of the mucin, increasing its hydrophobicity and thereby causing the mucin to contract.
Adhesion can be prevented by creating a deep fluid compartment between the lens and cornea so that mucin does not fill the space between the scleral lens and the cornea. However, a deep fluid compartment increases the prominence of the optic surface of the lens. This can make the lens uncomfortable to wear. A deep fluid compartment also increases the amount of fluid oxygen must pass through to reach the eye, and thus reduces the amount of oxygen that reaches the cornea. Thus a deep fluid compartment has disadvantages.
Suction can develop between the scleral lens and the cornea. During blinking, the scleral lens compresses against the eye and squeezes fluid out of the fluid compartment. If this fluid is not rapidly replaced when the lens decompresses, it functions like a one-way valve and becomes suctioned to the eye over time. This can be dangerous to the eye.
Fenestrations have been previously used to prevent suction. Ezekiel, D., “Gas permeable haptic lenses,” J. Br. Contact Lens Assoc. 6:158-161 (1983). However, fenestrations are often ineffective in preventing lens adhesion.
A scleral lens with channels is described in U.S. application Ser. No. 11/473,290 (published as US 2006/0290883), which is incorporated herein by reference in its entirety. A method of making such a scleral lens is described in U.S. Pat. No. 5,452,031, which is incorporated herein by reference in its entirety.